The present invention concerns lead-containing glasses with high X-ray absorptivity useful in the manufacture of image screens for cathode ray tubes with high operation potentials, (e.g., 30-60 kV), in particular for television, monitor and projection tubes.
For the production of image screens, glasses with a high permeability in the visible spectral range, with a high X-ray absorption and with a high resistance to brown coloration with regard to the undesirable browning effect which often results from electron and X-ray irradiation, are required.
It is known that, in cathode ray tubes, upon the impact of electrons on the phosphorus layer of the front plate or part of the image screen, there is generated a secondary emission of X-rays with a higher or lower intensity. Maximum permissible emission values were established at 0.5 milliroentgen per hour (mr/h) at a distance of 5 cm from the plate of the image screen. This is becoming even more critical a requirement, since the operating potential of the tubes is constantly increasing.
For example, higher operating potentials require television projection means which project the image onto a wall or a screen. Consequently, the emission of X-rays and electrons is very much higher than in conventional image screen devices. For this energy range, the .gamma. absorbers PbO, ZrO.sub.2, SrO, BaO and ZnO are the most effective elementary oxides.
Even though the high absorption ability of PbO is known, it has become the usual practice (see, for example, U.S. Pat. No. 3,464,932, U.S. Pat. No. 3,805,107, U.S. Pat. No. 4,179,638, U.S. Pat. No. 3,987,330, JP No. 8 100 9462, JP No. 8 100 9463, U.S. Ser. No. 122,505, U.S. Ser. No. 141,960 and GB No. 879,101), to provide adequate .gamma. absorption in alkali silicate base glasses by means of SrO, most often, however, by a combination of SrO+BaO in quantities of 0-20% by weight. ZrO.sub.2 is contained in most cases in amounts of 0-10% by weight. A linear X-ray absorption coefficient (.mu.) of 25 cm.sup.-1 at a wavelength of 0.6.ANG. is obtained in alkali silicate glasses by approximately 15% by weight of SrO. Higher coefficients are obtained by the addition of BaO and ZrO.sub.2 or ZnO, respectively. In addition, CeO.sub.2 is combined in quantities of 0.05-1.0% by weight, with amounts of up to 2% of TiO.sub.2 to increase resistance to discoloration.
Furthermore, it is known to use glasses with high lead contents for the cone and the neck of a cathode ray tube, but the same is not true for image screens. PbO is cited in the aforementioned patents with a maximum content of 0-5%, but if often absent entirely. Excessive amounts of this oxide were considered in the prior art as the sole cause of strong electron browning.
The aforementioned glasses pose the following difficulties if the requirement of .mu.(0.6.ANG.)&gt;30 cm.sup.-1 is to be satisfied.
(a) With strontium contents higher than 14% by weight, the formation of strontium silicate crystals may be observed during the cooling of the glass melt. A combination of Sr with Ba and Zr is again not free of a tendency to crystallize. This occurs particularly during reheating, e.g., for the fusing of the cone and the screen, both parts are heated to the processing temperature for approximately 1-2 min. in a narrow melting range.
(b) To obtain a satisfactory result in the fusing of the cone and the screen, in addition to comparable expansion coefficients, a similar viscosity behavior of the glasses at the temperature is also required. Due to the high .gamma. exposure of the cone, these glasses consist of at least 15-30% PbO, which causes a higher expansion and a relatively flat configuration of the viscosity curve with temperature. The alkali silicate glasses with SrO, BaO and ZrO.sub.2, on the other hand, are relatively "short" glasses, compared with the corresponding lead glasses.
(c) As a result of the high alkali and alkaline earth content, respectively, these glasses exhibit an acid resistance (DIN 12116) that is poor for a technical glass.
(d) These glasses strongly attack conventional, industrial glass tank materials, such as ZAC 1711, 1681 and Jargal M. This significantly restricts any large scale industrial production.
(e) The absorption coefficient cannot be raised over 50 cm.sup.31 1 without having to accept a severe deterioration of certain properties, such as chemical stability, crystallization, etc., so that the desire of industry for thinner front panes cannot be satisfied.